Update release post handbook page to revise file name instructions

Why is this change being made?

Currently, our yml file names are long and inconsistent. This makes it difficult to track files down when generating the release post. And long/inconsistent files names also don't serve process automation and visibility in the product system. This MR revises our release post item yml file naming convention to be more specific, brief and consistent across various content blocks.

Reference comment from release retro 13.5: #9165 (comment 441159159)

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.